In the past, the method for forming a vent slot in a thermoplastic closure was to first form helical screw threads on the periphery of a cylindrical male core. One or more vertical grooves would then be formed in the cylindrical male core which would traverse the helical threads. An insert was then placed in each of the vertical grooves so that the male core configuration would be the exact opposite of the inside of the closure. When the closure was subsequently axially stripped from the male core, the sharp corners on the threads, which were formed at the point where the threads met the sides of the insert, tended to deform in a downward direction. This downward deformation toward the open end of the closure tended to partially block the flow of pressurized gas which would pass along the underside of the thread as well as preventing the outward flow of pressurized gas through the vent slot. The slight deformation is frowned upon by the bottlers of carbonated beverages for it could create potential product liability should an injury occur due to improper venting of the pressurized gas within a container while a consumer removed the cap from the closure. The downward deformation of the closure thread is also hard to visually detect upon a rapidly moving conveyor belt, thereby making it difficult for a quality control operator to remove such closures before shipment. Besides the above-stated disadvantages, the shipment of such closures to the bottlers could degrade the quality of the closure manufacturer which could eventually discourage other potential customers from purchasing his product.
U.S. Pat. Nos. 4,392,005; 4,206,852; T4,427,126; and 3,556,334; and European Patent Application No. 79 30 0083.7 show examples of closures having a vent slot formed therein. U.S. Pat. Nos. 4,007,848 and 4,007,851 show the use of vent slots formed in the container rather than in the closure and onto which a metal closure is roll formed.
Now a male core has been invented which will overcome the deficiencies of the prior art. This male core will also form a thermoplastic closure having a thread which is less susceptible to being deformed as it is axially stripped from the male core.